The present invention is related to flow control in a communications network, and more particularly to credit chaining to control data flow and eliminate data loss.
Broadly, flow control in communications networks can be permission based or non-permission based. In a permission based network, a transmitter obtains permission to transmit from a receiver prior to transmitting data. The permission includes an indication that sufficient buffer space is available at the receiver for receipt of the data. Permissions may be periodically sent as a matter of course or may be in direct response to a request to transmit generated by the transmitter. In a non-permission based network the transmitter operates without explicit advance indication that sufficient buffer space is available at the receiver. Permission based flow control provides improved accuracy and control relative to non-permission based flow control, and in particular allows for a service that avoids loss due to network congestion.
One known type of permission based flow control is Quantum Flow Control ("QFC"). QFC supports an Available Bit Rate ("ABR") service in an Asynchronous Transfer Mode ("ATM") network by controlling the number of ATM cells buffered at each device in a connection. In particular, QFC is applied to each flow through an input port and associated buffer in a receiving device to assure that sufficient buffer space is available.
Implementation of QFC avoids ATM cell loss due to network congestion. However, devices that are primarily output buffered present a difficulty because flows from a plurality of input ports in the device may converge upon a single output port and associated output buffer. The calculations necessary to avoid cell loss in the output buffered device consequently involve a many (transmitters):1 (receiver) relation, rather than the 1 (transmitter):1 (receiver) relation in the case of a primarily input buffered device. It is known to alleviate this problem by partitioning each output buffer to create one partition for each input port. However, the use of reserved buffer partitions can result in inefficient underusage of overall buffer space because unused but dedicated partitions are unavailable for use by other ports and flows. Further, the requisite overall buffer size grows as the number of input ports increases, and hence may not scale well in some device configurations.